ACE is a virtual synthesizer, it doesn't read MODs at all. You can add samples to it but they're only as "additional help" because normally all sounds are generated in real-time there.

You can contact authors about format of the samples (here You can find their addresses: http://nb.atari.org/main.php?page=contactus ), however I am not sure if they can help because a year or 2 ago, I was asking them about sample-format of their FlexTrax editor (whether 8bit only, or maybe also 16bit samples in extended FLX mode?) and the guy answered he doesn't remember, after years. Hm.

Anyway I wish You luck and I hope You will upload Your ACE songs if they're ready.

Thanks for clearing that up yerzmyey - the OP used the word Tracker which to me means MOD files. Didn't realise ACE is a V-synth.... also quite impressed that a v-synth existed for the Atari! I must check that out myself